Since dust, causing air pollution, is discharged in combustion processes of industrial fields and thus adversely affects the atmospheric environment, many efforts to treat and control the dust are being made. Since filter dust collectors, used to remove such fine dust, exhibit very high dust collection performance regardless of the kind of dust or working conditions, most conventional electrostatic precipitators are being replaced by filter dust collectors. However, when conventional filter media for dust collection are directly applied to filter dust collectors at a high temperature of 250° C. or more in a state in which they are badly worn, problems may occur. Therefore, various alternatives for solving the problems are being researched.
For example, there is research on the use of an additional filtration effect due to the formation of a dust layer. This research is conducted based on the fact that, during the filtration of fine dust, solid particles are continuously deposited on a filter medium to form a dust cake layer, and the formed dust layer serves a layer for collecting other particles, thus obtaining an additional filtration effect. That is, after the initial dust layer is formed, as filtration continues, the filtration effect due to the dust layer, rather than the filtration effect due to the peculiar characteristics of the filter media, occurs. In this case, ultimately, the dust layer itself controls dust filtration performance, and the dust filtration performance is influenced by the structure of the filter media, the characteristics of dust (size, morphology, interaction between particles, etc.), filtration conditions (face velocity, dust concentration, operating temperature, etc.). In particular, since the surface structure and pore size of the filter media determines the characteristics of the initial dust layer, and the initial dust layer forms a subsequent dust layer and influences the filtration mechanism as filtration continues, the initial structure of the filter media ultimately controls the dust filtration characteristics. However, research on the formation of the dust layer has mostly been limited to fibrous filter media, and is intermittently conducted. In the case of inorganic filter media for purifying high-temperature exhaust gas, since they are manufactured, using special materials through special methods such that the form and properties thereof do not change, the structures of filter media differ from each other, and thus research on the formation of dust suitable for the structure difference is required.
Generally, the filter medium that has been applied to dust collection to date is a depth filtration type filter medium. However, this depth filtration type filter medium is problematic in that, since fine dust infiltrates into the filter medium, pressure drop increases, dust collection efficiency decreases, and the life span of the filter medium decreases. Therefore, currently, advanced countries are attempting to improve dust collection efficiency and dust removal efficiency and to increase the life span of filter media using a surface filtration method, which is a method of preventing fine dust from infiltrating into a filter medium by forming a porous surface layer on the filter medium.
Currently, methods of forming a porous surface layer include a coating method and a laminating method. The coating method includes foam (bubble) coating, aerosol coating, and electric coating. The laminating method includes film laminating and web layer laminating. Among the methods, the coating method is being mainly used as a method of maximizing the porosity of the surface of a filter medium. The reason is that the coating method has fewer technical restrictions than the lamination method. In particular, the foam coating is advantageous in that dust can be significantly removed because a surface layer is formed on the surface of a filter medium, and a large number of filter media can be produced in a process of manufacturing filter media. Further, the foaming coating is advantageous in that process chemicals are widely distributed on the surface of a filter medium, and are thus easily dispersed thereon because of the volume increase caused by foam. In addition, foam coating has advantages in that the amount of water that is used is decreased, the amount of wastewater that is discharged is decreased, the amount of process chemicals that are used is decreased, and productivity is improved. Therefore, foam coating is expected to be widely applied.
As points to be considered in foam coating, there are the structure, composition, weight, pretreatment of fabric, and properties of foam, such as viscosity, half-value period, rupture degree, absorbance of rupture foam, etc. It is possible to select suitable process conditions by making changes thereto. However, in foaming coating, a large amount of energy is consumed to vaporize water in a drying process after a wet process. Therefore, research on methods of reducing energy consumption in the drying process is required.
Conventional technologies using foam coating are described as follows.
Korean Patent No. 10-1992-9561 discloses a nonwoven fabric wallpaper and a method of producing the same, and discloses a method of producing a hygroscopic waterproof fabric by forming an adhesion layer, a micropore layer and a water repellent layer on one surface of spun and knit fabric; Korean Patent No. 10-2000-67428 discloses an air filter produced by coating surface of a nonwoven fabric with a resin foam obtained by air-blowing an emulsion obtained by uniformly mixing an emulsifier, such as an acrylic resin, an urethane resin, a polytetrafluoroethylene resin, a silicon resin, or the like, which is a water-soluble resin selected from low polymerized synthetic resins, with an acrylic thickener, a foaming agent, a foam stabilizer, a surfactant, an inorganic filler, active coal having a particle size of 100 mesh or less, zeolite having a particle size of 100 mesh or less and a cross-linking agent and then drying the nonwoven fabric coated with the resin foam; and Korean Patent No. 10-0367561 discloses a filter for filtrating fine dust, produced by coating a glass fiber fabric with a resin foam obtained by air-blowing an emulsion formed by uniformly mixing a water-soluble resin with an acrylic thickener, a foaming agent, a foam stabilizer, a surfactant, an inorganic filler and a cross-linking agent, and then drying the glass fiber fabric coated with the resin foam, and a method of producing the same. However, the above methods are advantageous in that a surface layer is easily formed on a filter medium, but are problematic in that, since an acrylic resin is used, thermal stability is not good, and thus fine dust included in medium and high temperature exhaust gas of 250° C. or more cannot be removed.
Accordingly, in order to improve thermal stability, Korean Patent No. 10-0367561 discloses a functional nonwoven fabric wallpaper, which has excellent flameproof and antibiotic properties and can maintain functionality semi-permanently, produced by applying a chemical foam coating agent, in which a flame retardant and an antibiotic are mixed, on the rear surface of a piece of nonwoven fabric using a foaming coating method, and a producing method thereof; Korean Patent Publication No. 10-2004-49278 discloses a ceramic filter produced by mixing at least one ceramic powder, selected from the group consisting of silicon carbide, alumina, sillimanite, kaolin, silica, titania and diatomite, with clay, a pore forming agent, a binder and a dispersant to prepare a slurry, supporting the slurry on a support to form a molded product, and then drying and sintering the molded product, and a producing method thereof; and Korean Patent No. 10-2004-49279 discloses a method of producing a multi-layered ceramic filter, comprising the steps of mixing at least one ceramic powder, selected from the group consisting of silicon carbide, alumina, sillimanite, kaolin, silica, titania and diatomite, with clay, a pore forming agent, a binder and a dispersant to prepare a slurry, supporting the slurry on a support to form a molded product, drying and sintering the molded product, applying an additional slurry, in which ceramic powder, a pore forming agent, a binder and a dispersant are mixed, on the interior or exterior portion of the molded product, and further drying and sintering the molded product, and a ceramic filter produced using the method. However, the above methods are disadvantageous in that, although thermal stability can be ensured, energy consumption at a heating temperature of 900˜1,300° C. in a sintering process is increased, and the dust removal efficiency of these methods is lower than that of foam coating. Meanwhile, Korean Unexamined Patent Publication No. 10-2002-22343 discloses a filter, which is mounted in an exhaust gas pipe located at the rear end of a diesel engine and serves to decrease the amount of particles discharged from the diesel engine, and in which a pad type filter medium is mounted in a quadrangular tube, and the circumference thereof is fixed with a net. However, this filter is also disadvantageous in that it is not flexible, and has low dust removal efficiency.